Wire-cut, electric discharge machining is employed to effect machining by discharging, between a wire electrode and a workpiece, charges stored in a capacitor. Conventionally, for example, as shown in FIG. 1, a transistor 1 is turned ON and OFF by drive pulses from a control circuit (not shown,) and when the transistor 1 is in the ON state, a capacitor 4 is charged by a DC power source 2 via a resistor 3. By turning ON a transistor 5, the stored charges are discharged between a workpiece 6 and a wire electrode 7. In FIG. 1, reference numeral 8 indicates a contact piece, 10 a diode for protecting the transistor 5 from a reverse breakdown voltage, 11 a stray inductance of a wire or the like, and 12 and 13 resistors for detecting the charging voltage of the capacitor 4 by dividing it.
With the wire-cut, electric discharge machining power supply unit shown in FIG. 1, the capacitor 4 is charged, by current flow from the workpiece 6 to the wire electrode 7, in a direction reverse from that in which it was charged by the DC power source, for example, as indicated by a waveform V.sub.C in FIG. 2(a). On this account, during discharging a current flows form the wire electrode 7 to the workpiece 6, resulting in a discharge current between the wire electrode 7 and the workpiece 6 containing a + direction component and a relatively large - direction component, for example, as indicated by a waveform I.sub.G in FIG. 2 (a). It is desired that the - direction component be as small as possible since it causes wear of the wire electrode 7 and leads to unstable cutting. One possible technique that is considered to meet this requirement is to isert a diode 20 in a backward direction with respect to the - direction component (as shown in FIG. 3) so as to cut the - direction component as indicated by a waveform I.sub.G in FIG. 2(b). With such an arrangement, however, after being discharged, the capacitor 4 is greatly charged in the polarity reverse from that in which it was charged by the DC power source, as indicated by a waveform V.sub.C in FIG. 2(b), and the total sum of currents required for recharging the capacitor 4 increases, thereby decreasing the cutting efficiency (the amount of cutting/current). It is also possible to connect a diode 30 in parallel to the capacitor 4, as shown in FIG. 4, so as to prevent it from being charged in the polarity reverse from that in which it was charged by the DC power source. With this arrangement, however, the gap current I.sub.G varies from the broken line curve to the solid line one in FIG. 2(c) and the discharge drags on, causing a substantial increase in the pulse width of the discharge current. In wire-cut, electric discharge machining, it is necessary that the pulse width of the discharge current (the pulse width of the + direction component) be as small as several microseconds or less. Therefore the arrangement of FIG. 4 introduces the defect that the discharge will become rather unstable.